We propose to investigate the role played by command interneurons in the control and execution of stereotyped behavior. This work initially maps the location of these interneurons within the CNS and provides by cinephotography details of the more complex behaviors evoked by their stimulation. Once refined neural maps are established, and the associated behaviors are understood, selected command fibers are to be subjected to morphological analysis by dye injections and to electrophysiological studies designed to determine how the units are used in ongoing behavior. These studies should help explain how the the organization and activity in a simple neural system provides for a particular behavior. Experiments are outlined to locate the site of a biological clock and to determine the neural mechanisms by which this central oscillator is entrained to the environment by photoreceptors. Other plans are proposed to determine the coupling which allows this biological clock to impose the daily cycles of activity within the animal. In the case of locomotor rhythms, the coupling is believed to be by way of command fibers already known to under lie the behavior. Finally, we will continue to study the evolution of a unique interneuron among a series of related species of animals. This work is designed to find the rules of the genetic retention or loss of either part or all of the functions of the identified cell.